Water supply, especially in urban areas, is one of the most important issues all
over the world. Tritium (3H or T) as a natural radioactive isotope of hydrogen
with a half-life (T1/2) of 12.26 years has been accepted as an efficient tool in
groundwater hydrology. By using an appropriate model, relevant hydro
meteorological and isotope data, a computer program which implements an
adequate recursive equation, the volume of the reservoir under the Galicica
Mountain that recharges the St. Naum Spring as well as the Mean Residence
Time (MRT) of the ground water has been determined and the respective values
are V=3.7 x 109 m3 and MRT= 6 years.
Keywords: volume of water reservoir, mean residence time, tritium
concentration.